Thermal bubble-forming inkjet printheads function by generating a certain amount of heat in a printing fluid contained in a nozzle chamber. This heat causes a bubble to form which eventually collapses as the fluid is forced through a nozzle. The collapse of the bubble then causes more fluid to enter the nozzle chamber for the same process to begin again.
The present Applicant has developed a range of thermal bubble-forming printheads. The Applicant's thermal bubble-forming printheads include those with suspended heater elements (as described in, for example, U.S. Pat. No. 6,755,509; U.S. Pat. No. 7,246,886; and U.S. Pat. No. 7,401,910, the contents of which are incorporated herein by reference) and those with embedded heater elements (as described in, for example, U.S. Pat. No. 7,377,623; U.S. Pat. No. 7,431,431; US 2006/250453; and U.S. Pat. No. 7,491,911, the contents of which are incorporated herein by reference). Other designs of thermal bubble-forming printheads, such as those available from Canon and Hewlett-Packard, will be known to the person skilled in art.
Inkjet inks for thermal bubble-forming printhead are typically water-based ink formulations comprising a pigment-based or dye-based colorant. A problem in virtually all thermal inkjet printheads is the build of solid deposits on heater elements used to heat the ink—a phenomenon known in the art as ‘kogation’. Kogation is problematic, because it reduces the efficiency of heater elements, but kogation may also cause nozzle failure. Inkjet inks comprising dispersions of pigments are usually more prone to kogation, although kogation can still be a problem in dye-based inks, and particularly any inks comprising polymer additives.
The problem of kogation has been addressed by various means in the prior art. In some inkjet printers, kogation is addressed through the use of non-ejecting drive pulses which disrupt kogative deposits on the heater elements sufficiently so that the deposits may be cleared. U.S. Pat. No. 5,440,330 and US2007/052761 describe the use of specific pulse voltages, pulse widths, pulse frequencies etc, which are useful in removing kogation. However, such approaches have limited success and may not be useful in removing all types of kogation.
EP-A-1302321 describes a printhead specifically designed to address the problem of kogation. Each ink ejection device comprises two chambers separated by a flexible membrane, whereby a first chamber is used to generate a bubble and a second chamber ejects ink. A pressure wave, which is transmitted from the first chamber through the flexible membrane to the second chamber, causes droplet ejection. An advantage of this arrangement is that a simple non-kogating fluid (e.g. water) can be used in the first chamber so that the heater element is free of kogation. However, the device is complex and necessarily occupies a larger area of the printhead than conventional bubble-forming chambers.
Other approaches to reducing kogation in the prior art employ additives in the inkjet inks. For example, US 2002/113842 describes the use of aldonic acid additives (e.g. gluconic acid) in pigment-based inks. Both self-dispersing pigments and pigments dispersed with polymeric dispersants are described. However, kogation was reported to be high in all cases in the absence of the aldonic acid additive.
EP-A-1279707 describes the use of organophosphonic acid additives for reducing kogation.
US 2008/066644 describes the use of amine additives in combination with two surfactants for reducing kogation.
U.S. Pat. No. 6,616,273 describes the addition of copper salts to inks, which delivers copper ions to heater elements to aid in the removal of kogation.
U.S. Pat. No. 4,790,880 describes the addition of crown ethers to inks in order to reduce kogation and improve print quality.
WO97/31984 describes pigment-based inkjet inks containing a vinyl silicone polymer, which is reported to reduce kogation.
It would be desirable to provide a new means for minimizing kogation of heater elements by inkjet inks, particularly pigment-based inkjet inks.